it doesn't need to be spelled out, as it has already been explained. however, there should not be a need for explanation. instant should mean instant. in this case, it doesn't, therefore, it causes issues for some.

does my toaster need to burn down the house,to remind me to put toast in it???

Hey, we're explaining the logic behind it... and it's the exact same thing that call forwarding from your landline phone company would do as well. Voip aims to mimic the landline as closely as possible, including it's shortcomings.